1. This invention relates to an exposure controller of a video camera.
2. Description of the Prior Art
Exposure control in an existing video camera was performed by comparing an video signal level with a reference level and, in response to an output resulting from the comparison, by controlling the open amount of an iris and the gain of an AGC amplifier. The circuit arrangement for the existing exposure control was an analog circuit. The analog circuit, however, not only increased the scale of the circuit but also had a poor temperature characteristic. It was also difficult for the analog circuit to control various setting values as desired.
It has therefore been proposed to digitally arrange the exposure control circuit of a video camera. In one exposure control method formerly proposed by the present inventor, a table, which stores gain setting data associated with data on a quantity of incident light, is prepared. The quantity of incident light is obtained on the basis of the level of an video signal, the current open amount of the iris and the gain of the AGC amplifier. The quantity of incident light obtained is given to the table, and the open amount of the iris and the gain of the AGC amplifier are controlled in response to the gain setting data output from the table.
FIG. 1 shows an example of the existing exposure control circuit. In FIG. 1, optical information given through a lens 51 and an iris 52 is formed into an image on a surface of CCD image pickup device 53. The iris 52 is opened and closed by an iris driving circuit 54. The electronic shutter speed of the CCD image pickup device 53 is set by a shutter speed setting circuit 55.
An output from the CCD image pickup device 53 is supplied through an AGC amplifier 56 to an A/D converter 57. The A/D converter 57 converts the video signal from the CCD image pickup device 53 into a digital form. An output from the A/D converter 57 is supplied to a video signal processing circuit 58 and to a level detecting circuit 59. The level detecting circuit 59 detects the level of the video signal. An output from the level detecting circuit 59 is fed to a light quantity data generating circuit 60.
The light quantity data generating circuit 60 obtains data on the quantity of incident light on the basis of an output from the level detecting circuit 59. When denoting the quantity of incident light by f.sub.IN, the iris gain by K.sub.IRS, the shutter gain by K.sub.SHU, and the AGC gain by K.sub.AGC, the input video signal level V.sub.IN is expressed by: EQU V.sub.IN =K.sub.IRS .multidot.K.sub.SHU .multidot.K.sub.AGC .multidot.f.sub.IN
Then the quantity of incident light f.sub.IN is determined by: EQU f.sub.IN =V.sub.IN /K.sub.IRS .multidot.K.sub.SHU .multidot.K.sub.AGC
The obtained data on the quantity of light is supplied from the light quantity data generating circuit 60 to a table 61. The table 61 stores gain setting values of respective elements (iris, shutter, AGC) associated with a quantity of incident light. In response to the data on the quantity of incident light from the light quantity data generating circuit 60, the table 61 outputs corresponding gain setting data of respective elements.
In response to iris gain setting data from the table 61, an iris driving signal is fed to an iris motor 54 via an iris driver 62 to control the opening amount of the iris 52. In response to shutter gain setting data from the table 61, the shutter speed setting circuit 55 controls the shutter speed. On the basis of AGC gain setting data from the table 61, the gain of the AGC amplifier 56 is controlled via a D/A converter 63.
In this manner, by performing exposure control in response to the data on the quantity of incident light obtained, the construction is simplified, and the exposure control condition can readily be changed by modifying data in the table 61. The existing exposure control circuit, however, involves such a problem that an error is produced in exposure control because of errors between the gain setting values of respective elements and actual gains of these elements.